Spectral information provided by Fourier-transform spectrometers (FTS) is currently used to diagnose chemical processes, detect pollutants, monitor atmospheric conditions in addition to many other uses supporting a variety of industrial activities. FTS, because of its attributes, appears to have become a tool of choice in the infrared (IR) and especially for applications requiring high detection sensitivity, high spectral resolution, wide spectral coverage and flexibility for system integration. In many applications, FTS is used as a spectral radiometer to determine the spectral density of energy contained in the radiation under study. However, in these applications, the instrument must be calibrated radiometrically and such calibration is rather cumbersome and especially when these instruments are operating at IR wavelengths. The main difficulty arises during operation in IR wavelengths since the output signal of the FTS contains parasitic radiation (self-emissions) in the IR which are generated by the instrument itself. Consequently, a peculiar calibration procedure involving two reference measurements is normally required in order to obtain and correct for this self-emission term in addition to the usual responsivity parameter. This self-emission term is generally uncontrolled and variable in time in existing Fourier-transform IR spectrometers (FTIR). Therefore, the best way to acquire radiometrically accurate spectra with these spectrometers is by doing frequent calibration measurements. The maximum accuracy is achieved when the two calibration measurements are updated for each target measurement. This increases the time required to generate a single calibrated spectrum by a factor of three. This requirement for frequent calibration presents a burden that reduces the efficiency and the applicability of these FTIR instruments, particularly for real time sensing. A paper entitled "Radiometric calibration of IR Fourier transform spectrometers: Solution to a Problem with the High-Resolution Interferometer Sounder" by H. E. Revercomb et al in Applied Optics, Vol. 27, No. 15, Aug. 1, 1988, is directed to a calibrated Fourier transform spectrometer known as the High-Resolution Interferometer Sounder (HIS). This particular HIS instrument performs in flight radiometric calibration, using observations of hot and cold blackbody reference sources as the basis for two-point calibrations. Another paper entitled "Differential detection with a double-beam interferometer", which is incorporated by reference, by J-M Theriault et al in the SPIE Vol. 3082 (pages 65-75) of Apr. 21, 1997 provides analysis of some methods used to radiometric calibrate single-beam and dual beam interferometers.
Various types of spectrometers exist such as the Michelson Interferometer described in British Patent 1,010,227 in which radiation from a source is collimated and the collimated beam is directed to a beam splitter, a semi-transparent plate at a 45.degree. angle to the beam, where part of the beam passes through the plate towards a mirror arrangement which reflects it back to the plate and then that plate reflects it towards an optical system which focuses that portion of the beam onto a detector. The beam splitter reflects a portion of the collimated beam from the source towards another mirror which reflects that portion back towards the beam splitter where it passes through the beam splitter towards the optical system that also focuses this portion onto the detector. One of the mirrors is movable to adjust the lengths of the beam paths so they can be made equal resulting in rays falling on the detector being in phase and producing a strong signal from the detector. If the movable mirror, however, is positioned so that there is a difference in length between the paths, the rays of a certain wavelength in one beam path will not be in phase with corresponding rays in the other beam path resulting in changes in the magnitude of the signal from the detector. A plot of the fluctuations of the signal from the detector against movement of the movable mirror from when the path lengths are equal is known as an "interferogram" and this can be used to deduce the wavelength distribution of radiation from the source. This British Patent is particularly directed to an arrangement for producing a difference in path lengths of the two beams from the beam splitter.
A number of different types of arrangements have been used to produce a difference in path lengths of beams from a beam splitter in spectrometers, several types of arrangements using a rigid pendulum structure with a moveable retroreflector (or retroreflectors) being described in U.S. Pat. No. 4,383,762 by Peter Burkert. U.S. Pat. No. 4,383,762 recognized that two-beam interferometers for measuring atmospheric transmissions when used in smaller spacecraft and/or measuring in low temperature ranges in cryostats require not only low weight and small dimensions but also extremely low heat generation as mentioned in the last paragraph in column 2. This U.S. Patent further states that "High complexity for low temperature measuring in cryostats is necessary for very weak radiation to prevent the inherent radiation of the measuring instrument from blanketing the source of radiation". Therefore, frictional losses in sliding guides, spindle guides and similar mechanical guides of moving parts should be minimized. In order to minimize those frictional losses, P. Burket proposed the use of a retroreflector in the path of one beam from the beam splitter which reflects that beam to a mirror and then back to the beam splitter and from there to the detector. That retroreflector is attached to the end of the rotatable rigid pendulum which accurately confines that retroreflector to a single plane during the swing of the pendulum from one position to another. The swing of the pendulum, as a result, produces a difference in path lengths of beams from the beam splitter that is used to determine the unknown spectrum of a source by evaluating the interferogram produced. This U.S. Patent also teaches several modified arrangements to this single pendulum including a double pendulum type with retroreflectors in each arm of the pendulum where one retroreflector is located in each beam path from the beam splitter to alter both beam path lengths. These pendulum retroreflectors arrangements minimise heat generated by frictional losses during movement of parts required to alter the path lengths of the two beams from the beam splitter. The friction in the pendulum bearing can also be minimised by using ball bearings or magnetic bearing as mentioned at the bottom of column 3 in U.S. Pat. No. 4,383,762.
U.S. Pat. No. 5,066,990 by H. Ripple describes another double pendulum type interferometer with mirror arrangements at each end of the pendulum's arms. Each mirror arrangement has two mirrors at right angles to each other and forms a retroreflector similar to those described in U.S. Pat. No. 4,383,762. Those mirror arrangements (retroreflectors) are located in each of the beam paths from the beam splitter and reflect these beams to a mirror which reflects the beams back towards the associated retroreflector where the beams are reflected back to the beam splitter and then to a detector. H. Ripple mentions that one problem that always arises is the compensation of differing temperature conditions in the interferometer as far as possible. H Ripple then indicates in the fifth paragraph in column 1 that in the interest of the smallest possible influence of this problem, "interferometers are usually provided with a thermostat, i.e. mounted in arrangements within which the greatest possible temperature constancy is sought with the most uniform temperature distribution possible". H. Rippel then states that in "practice, however, such systems are limited since the temperature regulation is limited according to the temperature conditions at particular points or in particular narrow partial regions within the instrument, so that certain temperature differences within the instrument usually cannot be completely avoided", H. Rippel describes a system wherein this temperature sensitivity is reduced by placing the semi-transparent mirror (a beam splitter) and the mirrors, those reflecting the beams back to the retroreflectors on the arms of the pendulum, onto a common carrier. That carrier is expediently manufactured of aluminum since it has high heat conductivity so that the beam splitter and mirrors on that carrier reach a largely corresponding temperature to considerably reduce the temperature sensitivity of the system.
U.S. Pat. No. 4,095,899 by George A. Vanasse describes another type of interferometer in which a first and a second beam splitter are optically aligned with the detector, the first beam splitter reflecting portions of an input beam to adjustable reflectors which reflect those portions back through the first beam splitter to the detector.
A second input beam is directed towards the second beam splitter which is optically aligned with the first beam splitter so that any output from the second beam splitter which is common to both input beams will be suppressed when they are combined at the second beam splitter. This arrangement can be utilised as a simple pollution detector or monitor if the first input beam, for instance, consists of radiation from an effluent (emitted by a stack, automobile, etc.) being studied after passing through an intervening atmosphere while the second input beam consists of radiation from an adjacent field of view which does not contain the effluent. Radiation common to both fields of view will then be suppressed in the interferometer and the resultant interferogram at the output will contain a structure due, for the most part, to only the effluent under study. U.S. Pat. No. 4,095,900 by R. E. Murphy is related to U.S. Pat. No. 4,095,899 in that both have a common inventor and it also provides an optical technique for suppressing unwanted background radiation from that originated by a target. Both of these last two patents are, however, directed to a structure that suppresses unwanted external background radiation and do not provide a structure to effectively suppress parasitic radiation (self-emissions) generated by the interferometer internally.